Conventionally, visible range semiconductor lasers have been adopted industrially for the applications of medical treatment and optical measurement and the like, and in recent years the applicable wavelength bands for them have expanded. The semiconductor lasers generally have the characteristics of narrow emission spectrum and high conversion efficiency, whereas the problem with them is that they are so sensitive to backward beam created by reflection that they are unstable in maintaining the properties against the reflected backward beams such as from the connection interface of optical fibers or from a substance being measured.
In order to maintain a stable operation of a semiconductor laser, it is essential that the reflected backward beam is prevented from returning to the beam emitter from which the beam was originated, and to do this an optical isolator capable of passing a forward beam but stopping a backward beam is interposed between the beam source and the object to be treated or measured so that the beam reflected from an optical fiber, for example, is stopped from returning to the beam emitter.
Now, it is necessary that the Faraday rotational angle is 45 degrees or so in order that such performance is achieved by an optical isolator. In particular, the beam entered in an optical isolator has its plane of polarization turned by 45 degrees by the Faraday rotator, and passes through an input polarizer and an output polarizer, which are respectively controlled to have particular angular positions. On the other hand the backward beam has its plane of polarization turned by 45 degrees in the counter direction by virtue of the non-reciprocation characteristic of the Faraday rotator, whereby its plane of polarization forms an angle of 90 degrees with lattice of the input polarizer so that there occurs no passage of the backward beam. The optical isolator therefore makes use of this phenomenon to allow passage of the beam in one direction only by prohibiting the passage of returning beam.
An optical isolator having such a function is composed of three main parts: a Faraday rotator, a pair of polarizers one installed on the beam entrance side and the other on the beam exist side of the Faraday rotator, and a magnet which impresses magnetic field in a direction of beam passage (beam transmission axis) of the Faraday rotator. In this kind of beam isolator, when beam enters the Faraday rotator a phenomenon is triggered such that the plane of polarization is twisted within the Faraday rotator. This phenomenon is generally called Faraday effect, and the angle by which the plane of polarization is twisted is called Faraday rotational angle, whose magnitude is denoted by θ and is represented by the following equation.θ=V×H×L In this, V is a Verdet constant, which is determined by the material and the wavelength of the beam used for the measurement. H is magnetic flux density and L is the length of the Faraday rotator (sample length).
As is understood from the equation presented above, in order to obtain a desired Faraday rotational angle θ in a Faraday rotator having a certain Verdet constant V, the greater the magnetic flux density H is that is impressed on the Faraday rotator, the smaller may the length L of the rotator be. On the other hand, the greater the length L of the rotator is, the smaller the magnetic flux must be, so that it is possible to reduce the size of an optical isolator making use of this relationship.
Since the determinants that determine the size of an optical isolator include Verdet constant V, which is determined by the kind of the material to make the Faraday rotator and the wavelength of the measurement beam, in addition to the magnetic flux H and the rotator's length L, it is important to develop a material which enables shortening of the Faraday rotator, in order to promote downsizing of the optical isolator.
IP Publication 1 discloses an oxide which contains ytterbium oxide in an amount of 30% or greater in terms of mass ratio, as a material that enables downsizing of the optical isolator. According to the description of this IP Publication 1, it is possible to downsize an optical isolator for use with wavelength of 320-800 nm, since if this oxide is used the Verdet constant V can be 0.050 min/Oe·cm or greater and the length of the Faraday rotator can be 50 mm or smaller, and at the same time the absorption of the beam having wavelength of 320-800 nm scarcely occurs.
However, in recent years, in the fields of medical treatment and industrial measurements where semiconductor lasers are used, there has been stronger calling for downsizing of the optical isolator which is used with wavelength bands of 600 nm-800 nm, and in order to answer this calling the conventional ytterbium oxide which enables Faraday rotator to have a length of 50 mm or smaller is not sufficient a material for Faraday rotator, and a material that enables a length of 11 mm or smaller is called for.
Conventionally, there have been known materials such as TGG (terbium gallium garnet) (Tb3Ga5O12) that are used to make a Faraday rotator used with wavelength of 600 nm-800 nm. The Verdet constant of TGG for use with wavelength bands of 600 nm-800 nm is as small as 0.27-0.50 min/Oe·cm, and in the case of an actually used TGG crystal, its Verdet constant is 0.46 min/(Oe·cm) or so for wavelength of 633 nm. When the Verdet constant is at the level of 0.46 min/(Oe·cm) it is necessary to use a relatively long optical passage in order to secure the function of an optical isolator, so that in consequence the problem arises that the dimension of the optical isolator becomes bulky. Incidentally, in the above, the term “min” stands for minute and is equivalent to one sixtieth of one degree of angle.
It may be thought to use glass containing lead, but such glass has a Verdet constant smaller than that of TGG in the wavelength range of 600-800 nm so that it is not a suitable material to make a Faraday rotator.